For a while now Google have offered personalised search which provides a custom ordering of search results based on your previous search behaviour. If you clicked on a particular site lots in the past and were signed in to Google.com, Google would remember the action and rank that site higher for you in future searches when you searched whilst signed in to the same account.
This personalised ordering can be a useful feature if you are trying to find a particular site which may not be ranking on the first page of the search results, however some SEO ‘professionals’ have made mistakes in the past where they have reported a site as ranking at number 1, only to be embarrassed when realised it was only ranking high in their personalised search results! Doh! Smart SEO people learned to make sure they were signed out of Google when tracking search results positions.
Yesterday Google announced that they will expand the personalised search functionality to offer custom ordering of search results even when NOT signed in to Google.com. This will work by storing an anonymous cookie on the client computer.
So if you are checking rankings of your pages and do not want to be embarrassed when you announced your site is ranking at number 1, only to realised that it is not for any of your colleagues, make sure you clear out your cookies first. Alternatively, you can turn off custom search results by following these instructions.